1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process.
2. Related Background Art
Recently, image forming apparatuses used as a hardware output machines for digital data information transmission via a data communication network and such information has been proposed like anything. As such image forming apparatuses, digital printers and digital copying machines are known.
Now, as an example of a conventional image forming apparatus, a digital printer will be schematically described with reference to FIG. 19.
FIG. 19 is a constructural view showing main portions of the digital printer. In FIG. 19, a photosensitive drum 1 as an electrophotographic photosensitive member is constituted by coating a photoconductive layer on a cylindrical conductive substrate and is supported for rotation in a direction shown by the arrow R1. Around the photosensitive drum 1, there are disposed, in order along a rotational direction thereof, a scorotron charger 2 for uniformly charging a surface of the photosensitive drum 1, an exposure device for reading an original and for forming an electrostatic latent image by exposure the photosensitive drum 1 in accordance with an image signal proportional to image density, a developing device 4 for forming a toner image by adhering toner to the electrostatic latent image, a corona transfer charger (transfer charger) 8 for transferring the toner image formed on the photosensitive drum 1 onto a transfer sheet (transfer material) P, an electrostatic separating charger (separating charger) 9 for separating the transfer sheet P to which the toner image is transferred from the photosensitive drum 1, a cleaning device 13 for removing residual toner from the photosensitive drum 1 after the toner image is transferred, and a pre-exposure (lamp) 30 for eliminating residual charge on the photosensitive drum 1.
By the way, after separated from the photosensitive drum 1 the transfer sheet P to which the toner image is transferred is conveyed to a fixing device 12, where the toner image is fixed to the transfer sheet to obtain a desired print image, and the transfer sheet P to which the toner image is fixed is discharged out of a main body of the image forming apparatus.
On the other hand, in a reader portion 18, an original 15 rested on an original glass stand 14 is illuminated by an illumination lamp 16, and light reflected from the original 15 is imaged on a photo-electric converting element (one-line CCD) 19 to convert the light into an electrical signal corresponding to image information. The light reflected from the original 15 illuminated by the illumination lamp 16 is imaged on the photo-electric converting element 19 through mirrors 17a, 17b, 17c and a lens 17d. The electrical signal outputted from the photo-electric converting element 19 is A/D-converted by an A/D-converter 21 into an 8-bit digital image data which is in turn logarithmically transformed in a black signal generating circuit 22 for changing luminance information to density information to obtain image density data.
The 8-bit digital image data signal formed in this way is inputted to a laser driving circuit 24. The laser driving circuit 24 is a well-known PWM circuit which modulates a light emitting time for ON/OFF of a laser diode in accordance with magnitude of the image density signal inputted.
For example, when the image data per pixel is inputted in a laser scanning direction as shown in FIG. 4A, a driving signal for ON/OFF of the laser becomes as shown in FIG. 4B. That is to say, ON duty of the laser driving signal when the image data is 00 hex is selected to 5% of one pixel scanning time, and ON duty of the laser driving signal when the image data is FF hex is selected to 85% of one pixel scanning time. A tone (dark/light) is achieved by effecting area-gradation within one pixel in this way.
Further, FIG. 6 shows general I-L property (driving electric current-amount of light property) of the laser. Since driving currents used for ON and OFF of the laser are I.sub.on and I.sub.off, respectively, the laser driving signal for the image signal of FIG. 4A becomes as shown in FIG. 4C, which is current for driving the laser by the PWM circuit.
By the way, the laser driving circuits are generally divided into the above-mentioned PWM circuit and a binary value laser driving circuit. As mentioned above, the PWM circuit serves to modulate to a pulse width signal corresponding to the time for lighting the laser diode in accordance with the magnitude of the inputted image density signal; whereas, in the binarizing circuit, the signal is converted into a two-step signal including special ON light emitting signal and OFF signal in accordance with the pixel size, and the converted signal is inputted to the laser driving circuit 24 so that the laser diode element is turned ON/OFF. As a typical method for binarization, there is a method in which the binarization signal is formed by an error diffusion technique and a dither technique on the basis of the image data, and, fundamentally, the time for generating the laser light is constant regardless of density. The difference is that the laser is emitted at low frequency for the pixel having low density and is emitted at high frequency for the pixel having high density.
The laser light driven and emitted in accordance with the image signal in this way is written on the photosensitive drum 1 in raster scan manner through a high speed rotating polygon mirror scanner 28 and a mirror 17f (FIG. 19), thereby forming the digital electrostatic latent image as image information.
Conventionally, many electrophotographic processes have been proposed as disclosed in U.S. Pat. No. 2,297,961, Japanese Patent Publication No. 42-23910 (1967) and Japanese Patent Publication No. 43-24748 (1968). In the general process, an electric latent image is formed on a photosensitive drum (recording member utilizing photo-electric material) by various means, and then the latent image is developed with toner (developer) to obtain a toner image which is in turn transferred onto a transfer material such as paper, and the toner image is fixed to the transfer material by heat or solvent vapor, thereby obtaining the copy image.
Further, various developing methods for visualizing the electric latent image by using the developer are already known. For example, there are magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, powder cloud development as disclosed in U.S. Pat. No. 2,221,776, fur brush development and liquid electrophoretic development. Among such developments, particularly, although the magnetic brush development using two-component developer mainly including toner and carrier has widely been put to practice, this development can provide good image relatively stably, but has disadvantage inherent to the two-component developer such as deterioration of toner and change in mixing ratio between the toner and the carrier.
In order to eliminate the above disadvantages, various developments using one-component developer consisting of toner have been proposed. According to this development, since control of the mixing ratio of the toner to the carrier is not required, the apparatus can be made more simpler.
In the above-mentioned conventional examples, when a corona charger is used as means for uniformly charging the photosensitive member, ozone and nitrate (discharged substance) form a film on the photosensitive member. If the film absorbs moisture in the air, surface resistance of the photosensitive member is decreased not to hold the charges of the electrostatic latent image (after exposure) including the image information data, with the result that the image is flown along the surface to distort the image information partially or totally, thereby flowing the image as if aqueous ink is flown. Similarly, when a corona charger is also used as a post charger, the similar disadvantage of the smeared image due to ozone will arise. Similar disadvantage will occur in the transfer charger and the separating charger.
In the past, in order to eliminate the above disadvantage, although a method in which ozone gas is removed by sucking air from the interior of a shield case of each charger by a fan has been adopted, the ozone gas cannot be removed completely, with the result that substances adhered to the surface of the photosensitive member cannot be prevented from affecting a bad influence upon the photosensitive member, and, thus, the smeared image cannot be prevented.
Further, in an apparatus having a long service life and permitting high speed image formation, rubber material separated from a pick-up roller and a conveying roller which supply and convey a transfer paper is adhered to the paper as offset which may be brought up to the surface of the photosensitive member to be adhered thereto. If the offset is gradually accumulated for a long term use, similar to the above-mentioned coated substance, the smeared image will be caused.
In order to eliminate the above inconvenience, there has been proposed a technique in which developer is adhered to the photosensitive member and such developer is brought to a cleaning device to enhance a polishing effect of the surface of the photosensitive member.
However, it is required that the developer be supplied not during the image formation and longer time is required for polishing after the supplying. If the longer polishing time is prepared, the preparation starting time of the image forming apparatus will become very long. Particularly, in the laser scanning and exposing system, it takes a long time for bringing the number of revolutions of the polygon mirror to the predetermined and constant number of revolutions in comparison with a conventional analogue exposing system.
Particularly when a-Si photosensitive member is used as the photosensitive member under a high humidity environment, an amount of ozone discharged during long term use is increased, and, thereafter, if the apparatus is left as it is for a long term, the ozone substrate and the discharged substrate will be adhered to the surface of the photosensitive member and will absorb moisture, with the result that, after the image forming apparatus is firstly powered ON, the smeared image will occur in initial images.
Further, when the digital data is written on the photosensitive member by the laser or LED spot exposure, the smeared image is particularly noticeable. When the dimension of the laser spot has a diameter of 50 to 70 .mu.m corresponding to 600 dpi, because of minute dots, even if the charges are disordered slightly, such disorder is integrated perceptionally as its collection and is felt further noticeably because of difference to the normal portion, thereby being recognized as great smeared image.
Further, in charge flying development and jumping development using one-component magnetic developer mainly utilizing electric field phenomenon, distortion of the electrostatic latent image on the photosensitive member is apt to be brought to the smeared image faithfully, and, particularly upon starting up of the image forming apparatus under the high humidity environment, the developer absorbs moisture while it is left as it is to reduce the surface resistance of the developer particles. As a result, since the charge amount of the developer to be held cannot be maintained to slightly decrease the developing efficiency, density reduction and smeared image are caused more noticeably. Such inconvenience is worsened in accordance with the moisture absorbed by the developing particles, and disadvantage occurs as reduction of density and/or smeared image in accordance with the humidity or water vapor amount in air.
Further, to eliminate the above disadvantages, there has been proposed a technique in which developer is adhered to the photosensitive member and such developer is brought to a cleaning device to enhance a polishing effect of the surface of the photosensitive member. However, in order to achieve the adequate polishing effect, it is required that a large amount of toner be supplied for the polishing or the toner be supplied frequently, with the result that, if the operator wants to obtain the small number of copies or prints, excessive developer will be consumed for the number of copies to be desired. That is to say, the cost for copy per one sheet will be increased or the toner will be consumed excessively.